The present invention relates to a semiconductor fabrication technology, and in particular, to an image sensor and a method for fabricating the same, and more particularly, to a complementary metal-oxide semiconductor (CMOS) image sensor and a method for fabricating the same.
With the development of image communication using the internet, demand of digital cameras is explosively increasing. Further, with an increase in distribution of mobile communication terminals with built-in cameras, such as personal digital assistants (PDA), International Mobile Telecommunications-2000 (IMT-2000), and code division multiple access (CDMA) terminals, demand of small camera modules is increasing.
In camera modules, charge coupled devices (CCD) and CMOS image sensors are widely used. A CCD has a complicated driving scheme and high power consumption. In addition, a CCD requires a large number of mask processes and the processes are complicated. Furthermore, a signal processor circuit cannot be implemented within a chip, so that it is difficult to realize on a chip. A CMOS image sensor includes a photodiode and a MOS transistor in a unit pixel and reproduces an image by detecting signals sequentially in a switching manner. Since the CMOS image sensor uses a CMOS fabrication technology, the CMOS image sensor has low power consumption and requires approximately 20 masks so that its process is much simpler than the CCD process which requires approximately 30 masks to approximately 40 masks. Since the CMOS image sensor can be realized on a chip with several signal processor circuits, it is considered as a next-generation image sensor.
Recently, higher density pixels are required to ensure competitiveness of CMOS image sensors. In order to implement high density pixels, the pixel size must be reduced. However, if the pixel size is reduced, the size of the photodiode is relatively reduced, and a fill factor, which is defined as an area occupied by the photodiode in a total pixel area, is reduced. If the size of the photodiode is reduced, a full well capacity, which is the number of signal charges one pixel can maintain, is also reduced and device characteristics are degraded. Thus, the area of the photodiode cannot be reduced without limitation.
Accordingly, as an effort to ensure the maximum well capacity within a finite area, there has been proposed a method that increases the area of the photodiode and decreases an interval of photodiodes, that is, an interval of adjacent pixels. However, the reduction in the interval of the photodiodes causes serious degradation in quantum efficiency (QE) and crosstalk characteristics of the image sensor, thereby leading to degradation in device characteristics.
As an effort to prevent the degradation of crosstalk characteristics, there been proposed a method that decreases the thickness of an epi-layer, and a method that separates the interval of the adjacent photodiodes by implanting impurity ions between the photodiodes. In the case of the former, the degradation of the quantum efficiency becomes more serious and, in the case of the latter, the width of the photodiode is relatively reduced, thereby causing the additional reduction of the maximum well capacity.
Conventionally, there have been proposed the above methods for reducing the inter-pixel crosstalk caused by diffusion of minority carriers, which are main factors of electrical crosstalk, but it can be seen that degradation of other main characteristics is caused.
Meanwhile, as the pixel size is reduced, it is required to additionally ensure the maximum well capacity. In order to increase the maximum well capacity in the finite photodiode region while maintaining a charge transfer characteristic, the photodiode is fabricated by performing an ion implantation process using low ion implantation energy. This is because the maximum potential depth within the photodiode is inversely proportional to the well capacity.
Therefore, in order to obtain a signal to noise ratio (SNR) and dynamic range meeting a level required in a small-sized photodiode, the ion implantation energy in the ion implantation process for fabricating the photodiode tends to be relatively lowered. However, these methods reduce the depletion region of the photodiode, causing the additional degradation in the quantum efficiency and crosstalk characteristic.